1. Field of the Invention
The present invention relates to an integrally fired, laminated electromechanical transducing element employing a piezoelectric or electrostrictive material.
2. Description of the Related Art
The integrally fired, laminated electromechanical transducing element used for an actuator, a piezoelectric transducer or an ultrasonic motor is fabricated by firing a plurality of ceramic layers, composed of piezoelectric ceramic or electrostrictive ceramic, integrally with internal electrode layers interposed between the ceramic layers.
This integrally fired, laminated electromechanical transducing element produces an inverse piezoelectric effect of generating a displacement upon application of a voltage thereto, and therefore a stress is generated between the internal electrodes and the ceramic layers. Further, another basic characteristic of this element is generating heat upon repeated application of a voltage thereto.
Thus, the following characteristics are required of the internal electrode layers:
1. low electric resistance and small loss of the electrical charge,
2. high heat conductivity and superior heat radiation characteristics,
3. superior antimigration characteristics,
4. a low rigidity and a low internal stress so as not to generate cracks or the like,
5. a high a bonding strength with ceramic, without separating in use, and
6. low cost.
With the conventionally integrally fired, laminated electromechanical transducing element, an Agxe2x80x94Pd alloy is widely used as an electrode material. Although high in conductivity and comparatively inexpensive, the reliability of Ag itself is low, as it has a low melting point of 960xc2x0 C. and easily migrates. On the other hand, Pd, though expensive, has a high melting point. Therefore, an Agxe2x80x94Pd alloy material can produce an electrode material having a high melting point with the migration suppressed (Japanese Unexamined Patent Publication No. 5-304043). Thus, the Agxe2x80x94Pd alloy material finds wide applications.
Although the migration is suppressed by adding Pd, the bonding between the electrode material and the ceramic material is not sufficient and, to cope with this problem, various measures have been taken as disclosed in Japanese Unexamined Patent Publications Nos. 5-304043 and 8-255509. Also, in spite of the fact that the migration is suppressed by adding Pd, the resultant higher cost poses a problem for industrial applications.
For these reasons, an integrally fired, laminated electromechanical transducing element is required which uses an inexpensive electrode material having the electrode characteristic at least equivalent to those of the Agxe2x80x94Pd alloy material.
(I) The present invention has been achieved in view of the problems of the prior art described above, and the object of the invention is to provide an integrally fired, laminated electromechanical transducing element employing an inexpensive electrode material having an electrode characteristic at least equivalent to an Agxe2x80x94Pd electrode or, in particular, an integrally fired, laminated electromechanical transducing element having a low rigidity of the internal electrode layers with only a small internal stress generated at the time of elongation or contraction of the ceramic layers (subject A).
According to a first aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element comprising an integrally fired laminate member including a plurality of ceramic layers of piezoelectric ceramics or electrostrictive ceramics and internal electrode layers interposed between the ceramic layers, wherein the main component of the internal electrode layers is a base metal having a rigidity not more than 160 GPa.
What should be noted in this invention is the use of a base metal having a specific property, i.e. a rigidity not more than 160 GPa as a main component of the internal electrode layers. As a result, the rigidity of the internal electrode layers, as a whole, of the integrally fired, laminated electromechanical transducing element can be reduced, which in turn can reduce the internal stress generated by the elongation or contraction of the ceramic layers when driving the integrally fired, laminated electromechanical transducing element. Further, the fact that the main component of the internal electrode layers is a base metal can reduce the cost as compared with the conventional Agxe2x80x94Pd material.
In the case where the Vickers hardness Hv exceeds 50 or the rigidity exceeds 160 GPa, the rigidity of the internal electrode layers as a whole increases and so does the internal stress at the time of elongation or contraction. This is liable to induce cracking or the like.
Specifically, a laminated actuator has such a characteristic that an assumed distortion X longitudinal to the direction of lamination is accompanied by a simultaneous transverse distortion of about one third, thereby generating a shearing stress between the internal electrode layers and the ceramic layers. Thus, the electrode material is required to have a low rigidity. The rigidity, though not definitely known, of the pure alloy composed of Ag and 30% Pd (hereinafter referred to as xe2x80x9cAgxe2x80x94Pd 30%xe2x80x9d) used as a conventional electrode material is estimated at about 160 GPa, a figure obtained as the product of 100.5 GPa (see Table 3), i.e. the rigidity of the pure metal Ag and 1.6, i.e. the hardness ratio of the Agxe2x80x94Pd 30% alloy to the Ag annealed at 500xc2x0 C. For preventing the adverse effect on the displacement performance and reliability of the actuator, the rigidity of the pure metal is required to be not more than 160 GPa, in which case the internal stress is estimated to be not higher than that of the Agxe2x80x94Pd 30% alloy.
In the case where the rigidity of the pure metal is not more than 160 GPa, the internal stress is estimated at not more than equivalent to that of the pure metal Agxe2x80x94Pd 30%. The base metals corresponding to this pure metal include Al, Cu, etc. Especially, Cu is desirable, as described later, as it has a volume resistivity as small as 1.7 xcexcxcexa9cm and a melting point of 1,083xc2x0 C. which is higher than and approximate to the sintering temperature 900 to 1,050xc2x0 C. of ceramics (refer to the ninth embodiment, described later).
According to a second aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element of which the displacement can be set to 0.06 to 0.15% when driven.
Specifically, the larger the displacement, the greater the internal stress of the electromechanical transducing element, resulting in a higher tendency to develop cracking.
Even in the case where the displacement is as high as 0.06% or more, however, the use of the specific base metal described above can suppress the internal stress and thus prevent the cracking.
The displacement of 0.15% or more reduces the strength of the ceramic layers themselves due to the repeated fatigue without regard to the electrodes, thereby undesirably shortening the service life thereof.
According to a third aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the average thickness of the electrode layers is desirably 1 to 8 xcexcm.
In the case where the average thickness of the electrode layers exceeds 8 xcexcm, the rigidity of the internal electrode layers increases for a greater internal stress when the element is driven. The average thickness of less than 1 xcexcm, on the other hand, undesirably both increases the resistance value of the electrodes and causes greater variations in the fabrication process.
According to a fourth aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the electrode forming ratio, i.e. the ratio which the portion formed with the electrodes represents of the total length of the internal electrodes exposed to the cutting section along the direction of lamination of the laminated member can be set to not less than 75%. The higher the electrode forming ratio, the greater the tendency of the rigidity of the internal electrode layers as a whole to increase. According to this invention, as described above, the inherent rigidity of the internal electrode layers can be reduced for a smaller effect of a rigidity increase due to a higher electrode forming ratio. Conversely, it is possible to reduce the electric resistance.
According to a fifth aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the main component of the electrode layers is desirably Cu, a Cu alloy or an oxide thereof comparatively low in cost. These materials also substantially meet the requirement of a base metal having a rigidity not more than 160 GPa (Table 3).
According to a sixth aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the electrode layers preferably contain at least selected one of Ca, Mg and Sr as a material.
Specifically, the electromechanical transducing element according to this invention, being of integrally fired laminate type, is fabricated by integrally firing a laminate member composed of an alternate arrangement of green sheets for forming ceramic layers and an electrode paste material for forming the electrode layers. Preferably, the electrode paste material contains Cu or the like as a main component of the electrode layers on the one hand and at least one of Ca, Mg and Sr on the other. In such a case, as described above, the electrode layers obtained by integral firing contain at least one of Ca, Mg and Sr.
The fact that the electrode paste material has a component containing at least one of Ca, Mg and Sr contributes to the following-described superior functions and effects for integral firing.
Specifically, in the presence of a component such as CaO, MgO or SrO containing at least one of Ca, Mg and Sr in the electrode paste material, the laminate member composed of the green sheets and the electrode paste material can be fired while at the same time preventing or suppressing the fusion of a mixture of Cu and the green sheets or increasing the melting point of the mixture. As a result, the segregation of the molten material containing Cu flowing into the ceramic layers can be suppressed, and therefore the ceramic layers are fired into a form which can exhibit the original superior performance thereof sufficiently.
According to a seventh aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the ceramic layers are preferably formed of PZT which is an oxide mainly having a perovskite structure of Pb(Zr, Ti)O3. This PZT exhibits very superior characteristics for the ceramic layers for the electromechanical transducing element.
According to an eighth aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the PZT preferably contains at least one of Mo and W. As a result, the sintering temperature of PZT can be reduced to facilitate the simultaneous firing with Cu.
According to a ninth aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element which can be used of for one of an actuator, a piezoelectric transducer and an ultrasonic motor. The cost of these products can thus be reduced and an improved performance obtained.
According to a tenth aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element which can be used for a fuel injection actuator for the injector. Specifically, the fuel injection actuator is required to be low in cost, high in responsiveness, high in durability and high in reliability, and these requirements can be met by the electromechanical transducing element described above.
(II) Further, the present invention is intended to provide an integrally fired, laminated electromechanical transducing element formed of an inexpensive electrode material having an electrode characteristic at least equivalent to that of the Agxe2x80x94Pd electrode, or in particular an integrally fired, laminated electromechanical transducing element having a superior antimigration characteristic (subject B).
According to an 11th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element fabricated by integrally firing a laminate member including a plurality of ceramic layers of the piezoelectric ceramics or the electrostrictive ceramics and internal electrode layers interposed between the ceramic layers, in which the internal electrode layers contain, as a main component thereof, a metal of which an oxide is stable in the atmosphere, and in which the value (A+B)xc3x97C is not more than xe2x88x9234,000 (kJ/mol)2, where A (kJ/mol) is the ionization potential per mol of the metal, B (kJ/mol) the thermal energy of evaporation and C (kJ/mol) the oxide formation energy of the metal.
What should be noted about this invention is the fact that the metal contained in the electrode layers is a metal of a value (A+B)xc3x97C not more than xe2x88x9234,000 (kJ/mol)2, in which A is the ionization potential per mol of the metal, B the energy of formation, and and C the oxide formation energy of the metal.
As a result, it is possible to fabricate an integrally fired, laminated electromechanical transducing element having a superior antimigration characteristic and a high durability.
For the property values and the antimigration characteristic of the metal material, refer to xe2x80x9cProceeding 31st. ECC (1981), pp.287-292xe2x80x9d describing the fact that the ionization potential and the thermal energy of evaporation are related to the antimigration characteristic. On the other hand, xe2x80x9cOMRON TECHNICS, Vol. 19, No. 4, pp.231-292,xe2x80x9d contains the description to the effect that the smaller the oxide generating energy, the superior the antimigration characteristic. Nevertheless, the truth has yet to be clarified theoretically due to the availability of very few related reports. It is common practice, therefore, to evaluate the antimigration characteristic by a water drop test or a similar experiment.
In view of this, the present inventors have come to realize that the migration requires that a metal, or a very small amount of a coexistent oxide of the metal, are ionized and jump out of the crystal lattices and that the metal ions remain without being oxidized at the same time. The inventors have also discovered that an antimigration constant Rm, defined as the sum of the ionization potential per mol of the metal and the thermal energy of evaporation, multiplied by the oxide generating energy, is negatively correlated with the antimigration characteristic (the time before the migration occurs) obtained by the water drop test. This relation, however, does not apply to the elements such as Au having a positive value of the oxide formation energy, i.e. the elements of which an oxide is very difficult to generate.
The property values of the conventionally used electrode material of Agxe2x80x94Pd 30% are not known. However, the antimigration constant Rm, if considered as an arithmetic mean of the atomic % of Ag and Pd, is about xe2x88x9234,000. In the case where Rm is smaller than xe2x88x9234,000, the antimigration characteristic is estimated to be at least equivalent to that of Agxe2x80x94Pd 30%. The corresponding base metals include Cu, Ni, Al, Mo and W. (refer to the ninth embodiment). Especially, the metal Cu is preferable as it has a volume resistivity as small as 1.7 xcexcxcexa9m (refer to the ninth embodiment) and a melting point of 1,083xc2x0 C. and higher than and approximate to the sintering temperature 900 to 1,050xc2x0 C. of ceramic.
According to a 12th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the volume resistivity of the metal contained in the internal electrode layers is preferably not more than 15 xcexcxcexa9cm. This can reduce the electrical resistance value of the internal electrode layers for improved characteristics of the electromechanical transducing element.
According to a 13th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which at least a part of the internal electrode layers is exposed to the side of the laminate member. Generally, the migration is facilitated by the exposure of the internal electrode layers. Nevertheless, according to the invention, the superior antimigration characteristic described above can be exhibited.
According to a 14th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the main component of the electrode layers is preferably Cu, a Cu alloy or an oxide thereof. The materials including Cu, a Cu alloy and an oxide thereof are comparatively low in cost and can easily achieve the value of (A+B)xc3x97C of not more than xe2x88x9234,000 (kJ/mol)2. As a result, an integrally fired, laminated electromechanical transducing element can be obtained which is inexpensive and superior in antimigration characteristic. Also, Cu, a Cu alloy or an oxide thereof has a low electrical resistance (refer to the ninth embodiment), and can improve the bonding strength between the electrode layers and the ceramic layers even with an increased electrode forming ratio of the electrode layers.
According to a 15th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the electrode layers preferably contain at least selected one of Ca, Mg and Sr.
Specifically, the electromechanical transducing element according to the invention, being of integrally fired laminate type, is fabricated by integrally firing a laminate member constituted of an alternate arrangement of green sheets for forming the ceramic layers and the electrode paste material for forming the electrode layers. The electrode paste material preferably contains, as a main component of the electrode layers, Cu or the like on the one hand and a component having at least one of Ca, Mg and Sr on the other hand. In this case, as described above, the electrode layers obtained after the integral firing process contain at least one of Ca, Mg and Sr.
The fact that the electrode paste material contains at least selected one of Ca, Mg and Sr can lead to the superior functions and effects described below at the time of integral firing.
Specifically, as long as a component such as CaO, MgO or SrO containing at least one of Ca, Mg and Sr exists in the electrode paste material, the fusion of the mixture of Cu and the ceramic material can be prevented or suppressed or the melting point of the particular mixture can be increased at the time of integrally firing the laminate member composed of the green sheets and the electrode paste material. As a result, the segregation caused by the molten object containing Cu flowing into the ceramic layers can be suppressed. Thus, the resultant fired ceramic layers can sufficiently exhibit the originally superior performance thereof.
According to a 16th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the ceramic layers are preferably composed of PZT constituting an oxide having a perovskite structure mainly of Pb(Zr, Ti)O3. This PZT exhibits a superior characteristic of the ceramic layers of the electromechanical transducing element.
According to a 17th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the PZT preferably contains at least one of Mo and W. This reduces the sintering temperature of PZT and facilitates sintering simultaneous with Cu.
According to an 18th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element usable for an actuator, a piezoelectric transducer or an ultrasonic motor. This can reduce the product cost and improve the performance of the product.
According to a 19th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element usable for a fuel injection actuator of the injector. Specifically, the fuel injection actuator is required to be low in cost, high in responsiveness, high in durability and high in reliability. These requirements can be met by use of the electromechanical transducing element described above.
(III) Further, the invention is intended to provide an integrally fired, laminated electromechanical transducing element made of an inexpensive electrode material having the electrode characteristic at least equivalent to that of the Agxe2x80x94Pd electrode or, in particular, an integrally fired, laminated electromechanical transducing element having a low electrical resistance, a low loss of the injected charge, a high heat conductivity and a superior heat radiation characteristic (subject C).
According to a 20th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element fabricated by integrally firing a laminate member including a plurality of ceramic layers of piezoelectric ceramics or electrostrictive ceramics and internal electrode layers interposed between the ceramic layers, in which the internal electrode layers contain, as a main component thereof, a base metal having the volume resistivity of not more than 15 xcexcxcexa9cm and the heat conductivity of not less than 50 W/mK.
What should be noted about this invention is that a base metal having such characteristics as the volume resistivity of not more than 15 xcexcxcexa9cm and the heat conductivity of not less than 50 W/mK is used as a main component of the internal electrode layers. As a result, the internal electrode layers having a low electrical resistance and a superior heat radiation characteristic can be obtained. Thus, the energy loss in the electrode layers can be reduced for improved characteristics of the electromechanical transducing element.
According to a 21st aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the volume of the laminate member can be not less than 500 mm3. In this integrally fired, laminated electromechanical transducing element, the larger the volume of the laminate member, the higher the tendency for the heat radiation characteristic to decrease. A large laminate member having a volume of 500 mm3 or more is liable to pose the problem of a reduced heat radiation ability. The integrally fired, laminated electromechanical transducing element according to this aspect, however, contains a base metal having a specific volume resistivity and a specific heat conductivity as a main component, and therefore a sufficient heat radiation characteristic can be secured.
According to a 22nd aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the sectional area of the laminate member can be not less than 18 mm2. In this integrally fired, laminated electromechanical transducing element, the larger the sectional area of the laminate member, the easier for the heat radiation ability to decrease. Especially for a large laminate member having a sectional area of 18 mm2 or more, the reduction in the heat radiation characteristic often poses a problem. The integrally fired, laminated electromechanical transducing element according to the invention, however, contains as a main component a base metal having a specific volume resistivity and a specific heat conductivity as described above and therefore can secure a sufficient heat radiation ability.
According to a 23rd aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element which can be used with an injection energy of 0.025 to 0.075 mJ/mm3 at the time of driving.
In this integrally fired, laminated electromechanical transducing element, the larger the injection energy for driving, the easier for the heat radiation ability to decrease. Especially in the case where the injection energy per unit volume in the driving process is not less than 0.025 mJ/mm3, the reduction in the heat radiation characteristic is often a problem.
Nevertheless, the integrally fired, laminated electromechanical transducing element according to the invention, containing a base metal having a specific volume resistivity and a specific heat conductivity as a main component, can secure a sufficient heat radiation ability. In view of the fact that the injection energy of more than 0.075 mJ/mm3 causes the heat generation of the element to exceed the heat radiation by the electrodes considerably, however, the ceramic is thermally degenerated undesirably for an actuator.
According to a 24th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the average value of the thickness of the electrode layers is preferably not less than 1 xcexcm. In the case where the average thickness of the electrode layers is less than 1 xcexcm, both the conductivity and the heat radiation ability decrease, thereby posing the problem of an increased energy loss in the electrode layers and an increased temperature of the element.
According to a 25th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the electrode forming ratio, i.e. the ratio which the portion formed with the electrodes represents of the whole length of the internal electrodes exposed to the section cut along the direction of lamination of the laminate member is preferably not less than 75%. This can further reduce the electrical resistance of the electrode layers and further improve the heat radiation characteristic.
According to a 26th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the main component of the electrode layers is preferably Cu, a Cu alloy or an oxide thereof. The materials including Cu, a Cu alloy and an oxide thereof are comparatively inexpensive and at the same time can positively meet the requirements of a base metal having specific characteristics such as a volume resistivity of not more than 15 xcexcxcexa9cm and a heat conductivity of not less than 50 W/mK (refer to the ninth embodiment).
According to a 27th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the electrode layers preferably contain at least one of Ca, Mg and Sr.
Specifically, the electromechanical transducing element according to this invention is of an integrally fired laminate type, and is therefore fabricated by integrally firing a laminate member formed of an alternate arrangement of the green sheets for forming the ceramic layers and the electrode paste material for forming the electrode layers. The electrode paste material preferably contains, as a main component of the electrode layers, Cu or the like on the one hand and a component having at least one of Ca, Mg and Sr on the other hand. In such a case, as described above, the electrode layers obtained after the integral firing process contain at least selected one of Ca, Mg and Sr.
The fact that the electrode paste material includes a component containing at least one of Ca, Mg and Sr as described above produces the following-described superior functions and effects in the integral firing process.
Specifically, in the presence of a component such as CaO, MgO or SrO containing at least one of Ca, Mg and Sr in the electrode paste material, the laminate member composed of the green sheets and the electrode paste material can be integrally fired while preventing or suppressing the fusion of the mixture of Cu and the ceramic material or increasing the melting point of the mixture. As a result, the segregation can be suppressed which otherwise might be caused by the molten material containing Cu flowing into the ceramic layers. In this way, the ceramic layers obtained can sufficiently exhibit the inherently superior performance thereof.
According to a 28th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the ceramic layers preferably are formed of PZT, i.e. an oxide mainly having the perovskite structure of Pb(Zr, Ti)O3. This PZT exhibits a highly superior characteristic as the ceramic layers for the electromechanical transducing element.
According to a 29th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the PZT preferably contains at least one of Mo and W. This reduces the sintering temperature of PZT while at the same time facilitating the simultaneous firing with Cu.
According to a 30th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element which can be used for selected one of an actuator, a piezoelectric transducer and an ultrasonic motor. These products can thus be reduced in cost and improved in performance.
According to a 31st aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element which can be used for a fuel injection actuator of the injector. Specifically, the fuel injection actuator is required to be low in cost, high in responsiveness, high in durability and high in reliability. These requirements can be met by use of the electromechanical transducing element described above.
(IV) Further, the present invention is intended to provide an integrally fired, laminated electromechanical transducing element having the electrode characteristic at least equivalent to that of the Agxe2x80x94Pd electrode by employing an inexpensive electrode material, or in particular to an integrally fired, laminated electromechanical transducing element having a superior bonding strength between the ceramic layers and the electrode layers (subject D).
According to a 32nd aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element fabricated by integrally firing a laminate member composed of a plurality of ceramic layers of piezoelectric ceramics or electrostrictive ceramics and internal electrode layers interposed between the ceramic layers, wherein the electrode forming ratio, i.e. the ratio which the portion formed with the electrodes represents of the total length of the internal electrodes exposed to the section cut along the direction of lamination of the laminated member is not less than 75%, and wherein the bonding strength between the internal electrodes and the ceramic layers is not less than 40 MPa.
What should be noted about this invention is that the electrode forming ratio is set to a value as high as not less than 75% and the bonding strength to not less than 40 MPa. As a result, an integrally fired, laminated electromechanical transducing element can be obtained which has a high bonding strength between the ceramic layers and the electrode layers and a superior electrode resistance.
The electrode forming ratio is a value calculated from the image obtained by observation under electron microscope or laser microscope of the laminate member mirror polished so as to expose the internal electrode layers in the section along the direction of lamination of the laminate member and represented by a value expressed as (B/A)xc3x97100%, where A is the total length and B the length of the whole electrode forming portion.
The bonding strength, on the other hand, is defined as a value obtained using, as a sample, a square metal plate having a side length of 3 to 10 mm and smaller than the laminate member which is attached to a hexagonal nut by an adhesive on each of the upper and lower surfaces of the laminate member, and a wire fixed in the holes of the hexagonal nuts is pulled at the rate of 0.5 to 2 mm per minute along the direction of lamination, so that the actual breaking load between the internal electrode layers and the ceramic layers is divided by the area of the metal plate.
In the case where the electrode forming ratio is less than 75%, the problem is posed that the electrical resistance of the electrode layers is increased considerably. Preferably, the electrode forming ratio is 100%. In the case where the bonding strength is less than 40 MPa, on the other hand, the problem is a reduced durability of the electromechanical transducing element In the case of the electromechanical transducing element according to the first embodiment, for example, an elongation of about 0.1% is caused along the direction of lamination by applying a voltage thereto. In the process, a tensile stress is generated in the neighborhood of the external electrode 31 shown in FIG. 1 along the direction of lamination in such a manner as to separate the internal electrode layers and the ceramic layers from each other, and cracking develops when this stress exceeds the bonding strength often resulting in shorting during operation.
The bonding strength required for preventing this cracking, as calculated by simulation, is 40 MPa.
According to a 33rd aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the average thickness of the internal electrode layers is preferably not more than 8 xcexcm. In the case where the average thickness exceeds 8 xcexcm, the rigidity of the internal electrode layers so increases that the internal stress is increased during the driving operation.
According to a 34th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the main component of the electrode layers is preferably Cu, a Cu alloy or an oxide thereof. The material such as Cu, a Cu alloy or an oxide thereof is comparatively inexpensive, low in electrical resistance and readily meets the requirements of both the electrode forming ratio and the bonding strength described above.
Specifically, an example is a case in which the ceramic layers contains Pb as a component. In the case where the conventional Agxe2x80x94Pd electrode is used, the bonding strength between the electrode material and the ceramic material is not sufficient. For securing a sufficient bonding strength, the portion where the upper and lower ceramic layers are bonded directly to each other without the intermediary of the electrodes is required to representing at least 25% of the whole, that is to say, the electrode forming ratio is required to be not more than about 75%. This reduced electrode forming ratio, however, poses the problem of an increased electrical resistance.
The use of Cu as an electrode material, on the other hand, can produce a high bonding strength while keeping a high electrode forming ratio as a compound is formed by Cu and Pb. As described above, therefore, the requirements for the electrode forming ratio of not less than 75% and the bonding strength of not less than 40 MPa can be readily met by forming the main component of the electrode layers of Cu, a Cu alloy or an oxide thereof.
According to a 35th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the electrode layers preferably contain at least selected one from Ca, Mg and Sr.
Specifically, the electromechanical transducing element according to the invention is of integrally fired laminate type, and therefore is fabricated by forming and integrally firing a laminate member including an alternate arrangement of green sheets for forming the ceramic layers and the electrode paste material for forming the electrode layers. In the process, the electrode paste material preferably contains, as a main component, Cu or the like on the one hand and a component including at least one of Ca, Mg and Sr on the other hand. In such a case, as described above, the electrode layers obtained by integral firing contain at least one of Ca, Mg and Sr.
The fact that the electrode paste material contains at least one of Ca, Mg and Sr leads to the following-described superior functions and effects in the integral firing process.
Specifically, in the presence of a component such as CaO, MgO or SrO containing at least one of Ca, Mg and Sr in the electrode paste material described above, the fusion of the mixture of Cu and the ceramic material can be prevented or suppressed or the melting point of the mixture can be increased when integrally firing the laminate member including the green sheets and the electrode paste material. As a result, the segregation or the like phenomenon which otherwise might be caused by the molten object containing Cu flowing into the ceramic layers can be suppressed. Thus, the ceramic layers can be fired into a state capable of fully exhibiting the inherently superior performance thereof.
According to a 36th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the ceramic layers are preferably composed of PZT which is mainly an oxide having the perovskite structure of Pb(Zr, Ti)O3. This PZT exhibits a quite superior characteristic for the ceramic layers of the electromechanical transducing element.
According to a 37th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element in which the PZT preferably contains at least one of Mo and W. As a result, the sintering temperature of the PZT can be reduced while at the same time facilitating the simultaneous firing with Cu.
According to a 38th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element which can be used for an actuator, a piezoelectric transducer or an ultrasonic motor. These products can thus be produced at a lower cost with an improved performance.
According to a 39th aspect of the invention, there is provided an integrally fired, laminated electromechanical transducing element which can be used for a fuel injection actuator for the injector. Specifically, a low cost, a high responsiveness, a high durability and a high reliability required of the fuel injection actuator can be achieved by employing the electromechanical transducing element described above.